1. Field of the Invention
This invention relates to improved silver catalysts for the production of ethylene oxide, their preparation, and their use in ethylene oxide processes.
2. The Prior Art
Materials consisting of silver upon a support are known to be useful catalysts for the production of ethylene oxide by the controlled incomplete oxidation of ethylene with molecular oxygen. A great variety of modifications have been proposed to improve the activity and selectivity of these catalysts. These modifications have involved, for example, the supports employed, the method of production, the physical form of the silver on the support and the addition of additives to the catalyst.
The alkali metals and their salts have been repeatedly proposed as additives for various silver ethylene oxide catalysts. Carter, in U.S. Pat. No. 2,125,333, issued Aug. 2, 1938, was among the first to disclose alkali metal addition. He specified the use of "small amounts" of alkali metals, including both sodium or potassium, in his silver catalyst. Later patents elaborated on this disclosure, but often with contradictory teachings. McNamee et al in U.S. Pat. No. 2,238,474, issued Apr. 15, 1941, disclosed that while addition of 100 ppm by weight to 24% by weight of sodium improved silver catalysts, these amounts of potassium had a detrimental effect on catalyst performance. Sears, Jr. et al in U.S. Pat. No. 2,615,900, issued Oct. 28, 1952, cited a large number of promoters useful in broad weight ranges, but made no distinction in the effectiveness of the various promoters. Saken, in U.S. Pat. No. 2,671,764, issued Mar. 9, 1954 disclosed the use of large amounts of alkali metal sulfates. Saken in U.S. Pat. No. 2,765,283, issued Oct. 2, 1956, proposed adding from 1 ppm by weight to 2000 ppm by weight of an inorganic chlorine compound to the catalyst support. Saken's inorganic chlorine compounds included the alkali metal salts of chlorine acids, especially sodium chloride. Carlson et al in U.S. Pat. No. 2,773,844, issued Dec. 11, 1956, disclosed a multistep silver deposition process for producing supported silver catalyst. Gould et al, in U.S. Pat. No. 2,799,687, issued July 16, 1957, disclosed that when from about 20 ppm by weight to about 1.6% by weight of inorganic halide (sodium chloride or preferably potassium chloride) are added as separate solid particles to a fluidized bed of supported silver catalyst, the halide acts as suppressant, inhibiting the catalyst activity. Hosoda et al in U.S. Pat. No. 3,144,416, issued Aug. 11, 1964, cited a number of promoter materials, but gave no critical limitations on their concentration. Kriger et al in U.S. Pat. No. 3,563,913 issued Feb. 16, 1971, generally disclosed the use of alkali and alkaline earth metals as promoters, listing specifically lithium with no reference to cesium, rubidium or potassium. He noted that these promoters are preferably added to the catalyst support before the latter is impregnated with the silver compound containing solution. Long in U.S. Pat. No. 3,575,888, issued Apr. 20, 1971, disclosed the use of aluminum oxide supports having a pore volume between about 0.15 and 0.30 m.sup.2 /gm and surface area below about 10 m.sup.2 /gm. Nielson in U.S. Pat. No. 3,702,259, issued Nov. 7, 1972 used certain organic amine solubilizing/reducing agents to product uniformly spaced, adherent, hemispherical deposits of metallic silver on catalyst supports. Nielsen et al in Belgium Pat. No. 793,658, issued July 4, 1973, disclosed critical ranges for alkali metal additions simultaneously deposited with the silver on the catalyst support, but did not disclose any critical relationship between support surface area and alkali metal concentration. Nielsen further stated that the addition of the alkali prior to the addition of the silver offered little or no benefit.
The prior art clearly recognizes that alkali metal compound addition changes, for better or worse, the character of a silver ethylene oxide catalyst. There is no recognition, however, that the deposition of potassium, rubidium and cesium prior to the deposition of the silver in critical amounts proportional to the support surface area produces superior ethylene oxide catalyst.